1. Field of the Invention
The present invention relates to a hybrid package, a glass ceramic substrate for the hybrid package and a composition for the glass ceramic substrate. More particularly, it relates to a highly reliable hybrid package wherein a glass ceramic substrate having a thermal expansion coefficient substantially the same as an aluminum nitride substrate is bonded to an aluminum nitride substrate, whereby formation of cracks is minimized, such a glass ceramic substrate and a composition for such a glass ceramic substrate.
2. Discussion of Background
Heretofore, it has been common to prepare a semiconductor circuit board of ceramics by forming a circuit on an inexpensive alumina (Al.sub.2 O.sub.3) substrate and mounting various elements such as semiconductor elements thereon. However, in recent years, reflecting the trend for high power of semiconductor elements, the quantity of heat generation of the elements tend to increase, whereby the above-mentioned Al.sub.2 O.sub.3 substrate is not necessarily satisfactory with respect to the heat dissipation property. To solve such a problem, it is conceivable to prepare a circuit board from aluminum nitride (A1N) having an excellent heat dissipation property instead of Al.sub.2 O.sub.3. However, A1N is far more expensive than Al.sub.2 O.sub.3, and the peripheral techniques such as metalizing are far behind. Therefore, it is not practical to employ A1N.
Under the circumstances, it has recently been proposed to improve the heat dissipation property and to reduce the cost by preparing a circuit board by a combination of an Al.sub.2 O.sub.3 plate and an A1N substrate and disposing the A1N substrate only below an element which produces a large amount of heat. (Japanese Unexamined Patent Publications No. 290158/1987 and No. 291158/1987). However, there is a large difference in the thermal expansion coefficient between A1N and Al.sub.2 O.sub.3 (A1N has a thermal expansion coefficient of about 45.times.10.sup.-7 .degree. C..sup.-1, and Al.sub.2 O.sub.3 has a thermal expansion coefficient of about 75.times.10.sup.-7 .degree. C..sup.-1). Therefore, a stress is likely to be created during the use due to the difference in the thermal expansion coefficient, whereby cracks are likely to form at the bonding portion, and in an extreme case, the A1N substrate may fall off. Further, when such a circuit board is sealed to form a package, the hermeticity will be unreliable.
It has also been reported to form a special metal layer at the bonding portion between the A1N substrate and the Al.sub.2 O.sub.3 plate to reduce the difference in the thermal expansion coefficient and thereby to improve the reliability (Japanese Unexamined Patent Publication No. 18687/1988). However, such a proposal still has a drawback that since the A1N substrate and the Al.sub.2 O.sub.3 plate which essentially differ in the thermal expansion coefficient are bonded to one another, when the assembly is subjected to an accelerated reliability test, for example, 1000 cycles of a temperature change between +125.degree. C. and -50.degree. C., defects such as peeling or cracks, are likely to form, whereby the hermeticity will be unreliable.